User operated drug delivery devices are as such known in the prior art. They are typically applicable in circumstances, in which persons without formal medical training, i.e., patients, need to administer an accurate and predefined dose of a medicinal product, such as heparin or insulin. In particular, such devices have application, where a medicinal product is administered on a regular or irregular basis over a short term or long-term period.
In order to accommodate with these demands, such devices have to fulfil a number of requirements. First of all, the device must be robust in construction, yet easy to use in terms of handling and in understanding by the user of its operation and the delivery of the required dose or medicament. The dose setting must be easy and unambiguous. Where the device is to be disposable rather than reducible, the device should be inexpensive to manufacture and easy to dispose. Moreover, the device should be suitable for recycling. To meet these requirements, the number of parts required to assemble the device and the number of material types the device is made from need to be kept to a minimum.
In particular for the purpose of dispensing of a dose, the drive mechanism of such drug delivery devices is adapted to exert an axially directed thrust on a piston being slidably disposed in a cartridge that contains the medicinal product. Due to the exerted thrust or pressure, the piston is displaced in distal direction, thereby expelling a well-defined dose of the medicinal product from the cartridge.
Since the drug delivery device should be light weight and cost efficient in production, most of its components, in particular the housing but as well as components of its drive mechanism are manufactured as injection molded plastic parts. Typically, during a dose dispensing procedure, at least some of the components of the drug delivery device might become subject to an at least slight elastic deformation.
Additionally, due to externally applied or internally generated mechanical forces, mechanical strain and tension may built-up during a dose dispensing procedure. Strain and/or tension, as well as elastic deformations of single or multiple components may remain in the drug delivery device also after dispensing of a dose. In practice it has turned out, that mechanical deformations and mechanical tension and/or strain may give rise to the development of so-called droplets to be observed at the tip of e.g. an injection needle or cannula being coupled to the cartridge in a fluid transferring way. It is further assumed, that due to relaxation processes or that due to vibration- or shock-induced motions of the drug delivery device, mechanical stress, tension and elastic deformations decay and vanish, e.g. due to component-inherent relaxation or inter-component vibrations.
This inevitable dissipation of elastic deformations and/or mechanical strain and/or tension may in turn lead to a further displacement of the piston into the cartridge, which, as a consequence gives rise to the unrequested development of droplets.
One approach to reduce generation of droplets comprises increasing the dimensions of components of the drug delivery device that are particularly subject to mechanical stress during dose dispensing. However, mechanically stabilizing components of the drug delivery device by simply enhancing their geometric dimensions, also the size of shrink marks, e.g. visible at the outside of the drug delivery device's housing may increase and may deteriorate the visual and haptic impression of the drug delivery device's housing. Such shrink marks are an inevitable consequence of an injection molding of plastic materials, which after injection molding is always subject to a particular shrinking process.